Area Forecast Discussion
Issued by NWS Eureka, CA
Issued by NWS Eureka, CA
945 FXUS66 KEKA 242151 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 251 PM PDT Tue Sep 24 2024 .SYNOPSIS...Interior cooling is expected on Wednesday, followed by a warming and drying trend later in the week into the weekend. Coastal low clouds and fog will return tonight and Wednesday, followed by strengthening northerlies and clearer skies later in the week. && .DISCUSSION...Another unseasonably warm day across the interior with temperatures in the mid 90s to 104F. Coastal areas have been just as warm as yesterday with afternoon temperatures reaching 70F at Crescent City and Arcata-Eureka airports and 81F at Fortuna airfield. A shallow marine air mass offshore over the waters has been edging closer to shore with only a few slivers of stratus and fog reaching the beaches and headlands of Mendocino county. Cooling is expected across the entire forecast area on Wednesday as a 500mb shortwave trough and surface front moving into the Pac NW. Based on NBM probabilities for 0.10 or more, precipitation chances are quite meager. Deterministic ECMWF and 50 member ensemble indicate a possibility (20% or less) for a few hundredths of rain, primarily for Del Norte county. Otherwise, only some light drizzle is possible as cool-damp marine air pushes onto the coast. Dense fog will also be a distinct concern tonight, especially after a couple days of offshore flow pushing warm air out over the chilly coastal waters. Ensemble means and deterministic guidance continues to indicate above normal 500mb heights and above normal 850mb temperatures returning toward the latter portion of the week and this weekend. Above normal interior temperature will most likely return Friday and over the weekend as northerlies increase offshore over the coastal waters, eventually becoming northeasterly over land areas with the development of a thermal trough. Looking at the National Blend of models (NBM), sure enough chance for 95F in the interior valleys increase to over 50% as early as Friday. Moderate heat risk is forecast to continue over the weekend into early next week with above normal interior warmth. Coastal warmth with highs around 70F or more will also be possible (25-35% chance) Fri and Sat, though deep column easterlies will be missing in this offshore case. Significant rain (a tenth of an inch) remains highly unlikely for the next 7 days. DB && .AVIATION...Satellite imagery showed extensive stratus and fog offshore over the coastal waters this afternoon. Fog and obscured skies have already reached Pt Arena, based on web cams. Timing arrival of LIFR vsbys and cigs (1/4SM vv002) into the coastal terminals remains uncertain. Current estimate is for fog to begin rolling onto the north coast this evening, after 02z, give or take 2 or 3 hours either side. Marine layer appears to remain shallow tonight before deepening on Wed. Thus expect only minor intrusion into the coastal river valleys until Wed or Wed night once northerlies ramp up offshore after frontal passage. DB && .MARINE...Weak front approaching the west coast will maintain a gentle wind regime across the waters tonight through Wed as thermal low pressure shifts inland. A mid period NW swell will build and peak around 8 ft at 14 seconds on Wednesday. Northerlies and steep waves ramp up again Wed night & Thu, strongest and highest downwind of Cape Mendocino. Potential for gale force northerlies with very large steep waves over 10 feet will increase Fri through Sat. DB && .FIRE WEATHER...Low daytime humidity from 10-20% has been occurring again today, however peak wind gusts have been less, up to 15-20 mph. Humidity is forecast to increase tonight into Wed for much of the area, though min RH`s from 20-25% are expected for most of zones 283 and 277 and portions of zone 264. Light to moderate westerly and southwesterly breezes are expected during the afternoon & evening in advance of 500mb trough. Localized peak gusts to 25-30 mph are over exposed terrain and channeled terrain are possible (10-20% chance) based on NBM/HREF 90th percentiles. Ensemble means indicate peak gusts from 15-25mph and will probably be more frequent and consistent. A warming and drying trend is forecast for the latter portion of the week into the weekend as stronger N-NE winds develop across the coastal ridges and exposed terrain. Elevated to locally critical conditions are possible. DB && .EKA WATCHES/WARNINGS/ADVISORIES... CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... None. && $$ Visit us at https://www.weather.gov/eureka Follow us on Facebook and Twitter at: https://www.facebook.com/NWSEureka https://www.twitter.com/NWSEureka For forecast zone information see the forecast zone map online: https://www.weather.gov/images/eka/zonemap.png